1. Field of the Invention
The present invention relates generally to means for packaging microelectronic devices and multichip modules. More particularly, the invention relates to improved flex-circuit solutions for SIMM and DIMM type memory modules and other modular electronic circuits.
2. Description of Related Art
High density computing is continually evolving to offer high performance devices with higher densities and smaller form factors. Servers are designed with these goals in mind and are key enablers for various computing applications. Servers contain, among other electronic components, microprocessors and memory modules, which generate considerable heat during operation. Currently higher performance servers make use of dual and quad core microprocessors. In turn, these microprocessors require memory modules with greater memory capacity for optimal performance.
Another important aspect of server design is that servers are often arranged in closely packed groups set in vertical racks that rely on forced air cooling for heat exhaust. So, air cooling is done for each server unit as well as the entire rack of server units. When multiple memory modules are tightly spaced together inside the tightly packed server chassis, the circulation of air as a cooling fluid becomes ineffective. Air can be restricted, leading to overheating of memory modules which in turn leads to premature failures including device failures and corrupt data streams.
The desirable attributes of the best memory modules include: high memory density, reduced thickness, reduced height, good thermal exhaust, good signal integrity, reliability, manufacturability and reduced cost. These characteristics are inter-related and the optimization of one may adversely impact the other; so, a balance must be found among these various characteristics to determine the most effective solution. The most effective solution may be relative to a given memory density. In other words, the most effective memory module solution at 2 GB or 4 GB may be different than the solution for 8 GB and 16 GB modules. Furthermore, the best DIMM solution for DDR2 DRAMs may be different than the best DIMM solution for DDR3 and DDR4 DRAMs since the standards are slightly different.
Increasing memory density inside a module can be achieved through stacking of chips. However, stacking of chips is accompanied by an increase in cost of the stacked chips. Also, stacking of chips is accompanied by greater heat that is concentrated between the stacked chips and is more difficult to dissipate which leads to an overall hotter module. Additional heat exhaust can be obtained through the use of heat spreaders or heat sinks. However, the addition of larger heat sinks and heat spreaders is accompanied by an increase in thickness. In turn, increasing thickness leads to a pressure drop between adjacent modules which impedes the flow of air and adversely affects cooling. The words heat spreaders and heat sinks are used interchangeably in this invention.
Thickness reduction can be achieved through the use of thin laminates and flexible circuits. However, flexible circuits can be more expensive than standard PCB based on FR4 materials. Furthermore, thickness reduction causes a mismatch between the thin laminate or the flexible substrate and the standard connector.
As previously taught by J. E. Clayton in a series of patents detailing the use of flexible circuits for memory modules (U.S. Pat. No. 6,665,190, U.S. Pat. No. 6,232,659, U.S. Pat. No. 6,091,145, U.S. Pat. No. 6,049,975, U.S. Pat. No. 5,731,633, U.S. Pat. No. 5,751,553, U.S. Pat. No. 5,708,297, U.S. Pat. No. 5,661,339, 2007/0211426A1, 2007/0212902A1, 2007/0212919A1, 2007/0211711A1, 2007/0212906A1, 2007/0212920A1) the design for the optimal thermal path puts the chip in a configuration that optimizes the heat path between the heat generation source and the metallic heat sinks and heat spreaders. However, when the chips are in an optimal heat path configuration, they may not be in an optimal electrical path configuration.
Flexible circuits exhibit many desirable attributes that lend themselves to solving many electronic packaging problems. Because they are constructed using thin flexible laminates, flexible circuits can be adapted into a large variety of three-dimensional configurations. In particular they are uniquely suited for joining two separate circuit components that involve repeated dynamic flexing motions such as when opening and closing cell phone, camera and notebook LCD displays.
Flexible circuits are also used in applications where reduced thickness or curved surfaces are important. Many mobile products produced today are made feasible by the unique characteristics inherent with flexible circuits. For background purposes, a fairly comprehensive description of flexible circuit technology, including construction methods, design and application specific examples may be found in a book authored by Dr. Joseph Fjelstad entitled “Flexible Circuit Technology” (3rd Edition—September 2006) the teaching of which is incorporated herein by reference in its entirety. Another reference on flexible circuits is “Coombs' Printed Circuits Handbook—Fifth Edition” by Clyde F. Coombs, Jr., the teaching of which is incorporated herein by reference in its entirety. Lastly, “Foldable Flex and Thinned Silicon Multichip Packaging Technology” edited by John W. Balde, is incorporated herein by reference in its entirety.
High performance computers, such as server and super computers, involve many dense, high frequency electrical connections where flexible circuits may be advantageously employed. Their thin uniform laminate thickness, ability to be fashioned with fine lined traces and small vias for layer-to-layer interconnections are better suited for higher frequency operation than traditional rigid printed wiring boards (PWB).
As clock frequencies increase with each succeeding generation of microprocessors, there is an increasing need to design circuit motherboards using techniques for controlled impedance and signal integrity. Usually this results in an increase in the layer count of rigid PWB circuit boards. Computer motherboards that could previously be designed with only 4 or 6 laminate layers now require 8 or more layers to properly route traces operating at higher clock rates. This may increase the cost of these special PCBs.
Using finer wiring patterns, flexible circuits can significantly reduce the number of required layers to form the same circuit functions. Rigid PWB motherboards are, nevertheless, presently required for mounting many hardware pieces such as power supplies, disc drives, fans, and component sockets and are therefore in no danger of being eliminated in the foreseeable future. However, the advantages of flexible circuits, as noted above, are leading many engineers to look for creative ways in which to include them in their designs. Examples developed by SiliconPipe are described by co-founder Dr. Fjelstad and illustrated on pages 32 and 33 of the reference text “Flexible Circuit Technology” cited above.
Although the use of flexible circuits in packaging semiconductors and consumer electronics is well known and offers the capability of high density interconnect signal stability at high frequencies, and flexible form factor (connecting sites that are not aligned), the technology based on flexile circuits is not without disadvantages. Flexible circuits present an overall cost disadvantage, they are inherently a non-rigid form factor and need structural members to be incorporated. Furthermore, the technology needs special design talent for high performance, reliability and operation at high frequencies. One of the challenges caused by the use of flexible circuits is that it reduces the thickness of the substrate to the point that it becomes incompatible with current standard connectors.
Multi Chip Modules (MCM) have a known form factor with known advantages and disadvantages. The advantages of thickness reduction in DIMM applications are increase air flow and space savings on the mother board. However, solving the thickness issues create other issues. Thickness reduction can be achieved by the use of Thin PCB, Rigid Flex, Flex circuitry (connected to standard PCB connector), or by using Flex circuit exclusively.
The thickness of a PCB can be reduced as is the practice of companies such as Eastern Company. These thin laminates can reduce thickness of the PCB but may not be good for high density modules and high frequency operations due to the limited number of layers they utilize and their dielectric properties. However, when thin PCB laminates are used a transition between the thin laminate and the wider DIMM connector is needed.